doc: update heap references according to auto heap calculation

Add and exchange global heap configuration CONFIG_HEAP_MEM_POOL_SIZE
against configuration used for auto heap calculation
CONFIG_HEAP_MEM_POOL_ADD_SIZE_ZBUS.
Add CONFIG_HEAP_MEM_POOL_ADD_SIZE_ZBUS as related configuration

Clarify that HEAP is used for zbus in a whole

Consistent naming 'Zbus'/'zbus' to 'ZBus'

Signed-off-by: Matthias Plöger <matthias.ploeger@phoenixcontact.com>